The accurate localization of hollow probes, such as needles and catheters, by ultrasound, particularly those needles and catheters used in diagnostic and surgical procedures, is described in several articles (for example, Ultrasonics, 1988, vol. 26, pp 27-30; Journal of Ultrasound in Medicine, 1990, vol. 9, pp 243-245; and Medical Electronics, April, 1995, pp 64-65) and in several patents (for example, EP A 0083973; EP A 0453251 U.S. Pat. No. 5,095,910; GB A 2157828). This technology arose from the need for fast, accurate location of the tips of needles and catheters. For example, in aspiration biopsy, it is important that the tip of the biopsy needle be accurately placed into the area of interest before a tissue sample is collected.
A disclosure relating to a method for enhancing the visibility of a hollow probe to a Doppler ultrasound imager is described in U.S. Pat. No. 5,549,112. This disclosure provides an apparatus comprising a hollow tubular probe, such as a needle, which is adapted for insertion into body tissue, the hollow probe being provided with a transducer which is substantially mechanically isolated from the probe and coupled to a fluid column within the probe, the transducer being arranged to generate a longitudinal oscillation of said fluid column at a sub-ultrasonic frequency which enhances the visibility of the probe tip to Doppler ultrasound imaging. One means by which the transducer might be mechanically isolated from the hollow probe is to couple the transducer to a column of air or gas in a flexible connecting tube, the internal lumen of which communicates with the bore of the probe.
One problem with the disclosure described in U.S. Pat. No. 5,549,112 is that the optimal visualization of the tip of hollow probes of different lengths and diameters may require the generation of longitudinal oscillation of different frequencies and/or amplitudes. The colour of a region of a Doppler ultrasound image is dependent on the velocity of the corresponding region of tissue and this velocity will vary with the frequency and amplitude of the oscillation of the end of the fluid column at the probe tip. In turn, the frequency and amplitude are dependent on both the mechanical properties of the tissue being vibrated by the fluid column and on the mechanical properties of the connecting tube and/or the probe itself.
Accordingly, it may be necessary for the user of such a device to adjust, recalibrate or reprogram the frequency delivered by the transducer each time that a probe system, consisting of a connecting tube and probe, with a different length and/or diameter and/or material is utilized.